This invention relates to a vent cap used in connection with a hydraulic apparatus such as a hydrostatic transmission (HST) or an integrated hydrostatic transaxle (IHT), where the HST is in a common housing with output gearing, a differential, output axles and the like. The general design of an IHT is known, and is disclosed in U.S. Pat. Nos. 5,314,387 and 6,185,936, the terms of which are incorporated herein in their entirety. Such IHTs are generally used with lawn and garden tractors and similar low speed vehicles. The invention could also be used with a wide variety of other hydrostatic applications, including industrial applications. It is described in connection with an HST or IHT used in a vehicle simply for convenience and simplicity, and such description should not be read as a limitation on the scope of the invention.
An HST or IHT generally has an oil sump in which various components, such as the hydraulic pump, motors and/or gears, are located. During operation, the operating oil will heat and expand, and so a reservoir is provided into which the expanded oil may flow. When the oil cools, it will again flow from the reservoir to the main sump. Such a reservoir may be internal to the main sump either as a separate formed structure therein or it may be an external structure connected to the main sump through a hose or the like. It may also take the form of simply an open space at the top of the sump.
In any case, the reservoir must be open to the atmosphere to permit air flow in and out and thus a vent is used to permit such air transfer. A major problem with prior art designs is that the vent opening also permits water to seep into the reservoir and thus into the oil supply. As is known, water can cause significant problems in the operation of hydraulic equipment, including degraded operation, cavitation and eventually damage and failure. Known designs have an opening to the reservoir vent with some type of cap structure mounted thereon to keep water out; however, such designs still permit water to seep into the reservoir through capillary action or otherwise.
Such hydrostatic drive apparatus are frequently used in situations where exposure to water cannot be prevented. For example, an IHT used in a lawn and garden tractor will frequently be subject to water spray, either accidental or intentional. These units are often cleaned with a hose, so that a spray may be inadvertently directed onto the vent for the oil reservoir.
The present invention solves these problems through an improved vent cap design, as will be disclosed herein with respect to the figures described below. A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and is indicative of the various ways in which the principles of the invention may be employed.